Apparatus for the detection of foreign bodies



Oct. 28, 1958 E. N. SHAWHAN 2,858,505

APPARATUS FOR THE DETECTION OF FOREIGN BODIES Filed Sept. 18, 1953 w OSGI LLATOR 8: CONTROL FIG.2.

\ IN V EN TOR. /24 ELBERT NEIL SHAWHAN ATTORNEYS APPARATUS FOR THE DETECTION OF FOREIGN BODIES Elbert Neil Shawhan, Newtown Square, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey -Application September 18, 1953, Serial No. 380,901

11 Claims. (Cl. 324-41) This invention relates to apparatus for the detecting of foreign bodies and, in particular, to the detection of metal particles in non-conducting materials such as waxes.

Various hard waxes are shipped by the producer t the customer in flake form and utilized in the final product without remelting. The flakes are formed by breaking up a continuous sheet of wax cut from a steel cylinder which rotates partly submerged in a tank of molten wax and becomes coated as'a film of wax is chilled on its surface. It is important to insure that the wax flakes thus produced are free of metal chips and the present invention in particularly designed to detect metal particles in wax flakes. However, as will become apparent hereafter, the invention is not limited to this but has broader aspects of applicability in other situations wherein similar problems arise. One source of difficulty in detecting the presence of metal chips in a material. in flake or other solid form such as wax involves the fact that air has a quite substantial magnetic susceptibility, and if the chips of wax or other material do not feed uniformly but abnormally pile up, as is inevitable, the displacement of air by the chips presents to an electrcial detecting apparatus a picture similar to the removal of a substantial amount of metal corresponding to more than the amount which would be present in a chip to be detected. A detecting system which is accordingly merely responsive to a change in magnetic susceptibility will fail to differentiate between the actual presence of metal and a completely harmless situation which simulates a change of metal content in the field of detection.

Variations in moisture content also constitute a disturbing influence. Vibration due to gearing and the whipping of a conveyor belt are also involved as well as shocks resulting from manipulation of a damper to divert a stream of chips from one container to another.

Various detectors of metal have been suggested in the prior art but none appears to be adequate in situations such as that discussed above.

The broad object of the present invention is the provision of a detector for foreign bodies, and, in particular, conducting bodies such as particles of metals in an irregular flow of an insulating material such as wax. In accordance with the invention, detecting means are provided which differentiate between conditions which should give rise to a signal and spurious conditions which are harmless but which if they were detected would result in unnecessary shutdowns and delays of operation.

Other objects of the invention particularly relating to details of construction and operation will become apparent from the following description read in conjunction with the accompanying drawing, in which:

Figure 1 is a wiring diagram showing the electrical details of a preferred form of apparatus provided in accordance with the invention; and

Figure 2 is a diagram, partly in section, illustrating certain mechanical arrangements of the apparatus and also showing in graphical form the signals which are produced by both metal particles and spurious disturbances.

Referring first to the mechanical arrangements illustrated in Figure 2, 2 represents a supplying hopper for wax chips, or the like, which provide the chips to a belt 4 which is trained over pulleys and driven by a motor 6, the chips W being discharged into a receiving hopper 8. The detecting means immediately physically associated with the chips on the belt comprise the exciter coil 10 and a pair of detector coils 12 and 14 which are arranged serially in the direction of travel of the belt. Each of these coils is wound upon a rectangular form elongated in a direction transverse to the belt, the various coils being fixedly and rigidly spaced with respect to each other and providing, as will appear, a system which may be balanced to provide a suitable background condition. As will be noted from Figure 2, the detecting coils 12 and 14 are symmetrically arranged with respect to the exciting coil 10, there being provided slight adjustability of one of the coils 12 or 14 to secure initial balance.

Referring now to Figure 1, there is indicated at 16 an oscillator which may take various known forms but which desirably comprises a stable oscillating circuit with an amplifier, desirably of a push-pull type to minimize the.

output of second harmonics. A wide range of frequencies may be utilized but it has been found for the purpose of detecting metal particles in wax chips a frequency of the order of 11 kilocycles per second is very satisfactory. The output of the oscillator is fed through a transformer 1S to the exciter coil 10 in series with which there is a condenser 20 chosen to provide a resonant circuit with the coil 10 to provide maximum current flow in the coil.

f The coils 12 and 14 have mutual inductance with the coil 10 and are connected in series opposition and in association with a balancing circuit comprising the respective condenser arrangements 22 and 24 and the resistance arrangement indicated at 26 including the adjustable potentiometer 28. A condenser 30 across the coil which is most remote from ground partially compensates for the capacity between the leads and ground. The capacities indicated at 22 and 24 are adjustable to provide difference in phase of the outputs to the voltage divider arrangement 26, 28. The amplitudes in the two paths of the circuit are balanced by adjustment of the potentiometer 28. The balancing in both phase and amplitude is desirably so carried out as to secure a minimum signal to the amplifier comprising the pentode 32 in conventional connections including the plate impedance 34 tuned to the oscillator frequency. Zero input signal is not required but it is only necessary that the direct pickup from the exciter be reduced to a point at which the amplifier is not overloaded. To assist in balancing there is provided at 36 a triode in a square law detecting circuit receiving the output from the amplifier, and indicating the conditions existing on a meter 38.

The output signal from the amplifier is delivered at 4-6 to the arrangement of triodes 42 and 44 including the phase shifting arrangement of a condenser 47 and a variable resistor 45 to provide at the output of the arrangement through condenser 46 a signal having the optimum phase. The anode and cathode resistors of the triode 42 are equal and the arrangement is not designed to provide amplification. The signal from the cathode of triode 44 passes through condenser 46, which blocks D. C. residual unbalance voltage, to the phase-sensitive detector provided by a pair of diodes 50 and 52 connected as disclosed particularly in my Patent No. 2,559,173, dated July 3, 1951. In particular, the condenser 46 is connected through the respective equal resistances 54 and 56 to the anode of a diode 50 and the cathode of a diode 5'2. Through the anode of diode 50 and the cathode of diode 52 are fed synchronizing signals through connections 58 and 60 from the oscillator 16, the signals being 180 out of phase. The output from the phase detector 48 is delivered through connection 62 from the cathode of diode 50 and the anode of diode 52. The phase-sensitive detector 48 is of synchronous type and the operation is fully described in the patent referred to above. In brief, there is provided by the arrangement a signal appearing across the condenser 61 which varies with the phase relationship of the signal to the amplifier pentode 32 with respect to the oscillator output. It is this phase shift which is utilized for detection purposes. In particular, this provides considerable discrimination against moisture variations in the material undergoing detection.

The signal appearing at connection 62 is fed through condenser 64 to the conventional two stage amplifier comprising the triodes 66 and 68 and the output from this amplifier is delivered to a cathode follower circuit including the triode 70. The two stage amplifier comprising the triodes 66 and 68 is broadly tuned to the fundamental of the signal voltage which appears at its input for a particular belt speed. For example, this signal voltage may have a fundamental frequency of around three cycles per second. The output from the cathode follower is delivered at 72 to a pulse selector circuit which will be more fully described hereafter. This circuit comprises the diodes 74 and 76 having polarity connections as indicated and connected to the input line 72 and across a condenser 78. One terminal of the condenser 78 is connected to ground through a resistance 80. The other terminal is connected to the series arrangement of a resistance 82 and a condenser 86, the latter being shunted by a high bleeder resistance 84. The terminals of the condenser 86 are connected to ground through the respective diodes 88 and 90 having the polarities indicated. Condensers 78 and 86 are of relatively large capacity, each, for example, having a capacity of'0.5 microfarad, while resistance 82 has a fairly high resistance value as compared with the forward resistances of the diodes which may conveniently be of crystal type. Resistance 80 has a relatively low value in comparison with resistance 82. Connected across the output of this pulse selector circuit is a potentiometer 92 the adjustable contact of which is connected to the control grid of a thyratron 94. The cathode of the thyratron 94 is maintained somewhat positive with respect to ground by its connection at 96 to the arrangement of resistances and 97 between the positive potential supply line and ground. The anode of the thyratron 94 is connected through currentlimiting resistance 98, relay coil 100, and the contacts at 102 of another relay to the positive potential supply line.

The contacts 102 are operated by a relay coil 104 which is connected to the ungrounded alternating current supply terminal 106 and to ground through a manually operable switch 108, one of the terminals 106 being grounded. The contacts of the relay indicated at 110 are connected between the ungrounded supply terminal 106 and to the coil of a power relay 112, one terminal of which is grounded. A signal lamp 114 may be arranged in parallel with the relay coil 112. This relay controls the contacts at 116 which are arranged in series with supply terminals and a signal and control unit which is diagrammed at 118 and which, depending upon what is desired, may be arranged to provide visual and/or audible signals and may also be arranged to stop the operation of the apparatus, including the progress of the belt 4, by deenergizing the motor 6.

In the operation of the device, particular interest is centered on the distinction between signals due to the presnce of metal particles and due to abnormal accumulations of wax chips. These two causes give rise to opposite phase shifts of the signals fed to the pentode 32 and produce pulses of opposite sign at the condenser 61 and on line 62. The amplified signals thus resulting at 72 are pictured at the bottom of Figure 2.

If a metal particle passes successively the detecting 4 coils 12 and 14, the resulting signal at 72 plotted against the position of the metal particle is as indicated at A, passage by the coil 12 providing a negative pulse 120 and passage by the coil 14 providing a positive pulse 122. As the particle passes the space between the coils the signal is essentially zero.

In contrast with what has just been stated, an accumulation of chips representing displacement of the relatively susceptible air results in a signal such as indicated at B, involving a positive pulse 124 as the chip accumulation passes the coil 12 and a negative pulse 126 as the chip accumulation passes the coil 14.

The purpose of the pulse selector circuit is to provide firing of the thyratron 94 when the condition A exists and to prevent firing when the condition B exists; in other Words, the selector distinguishes between the order of arrival of positive and negative pulses.

Considering first the condition B, the positive pulse 124 effects charging of the condenser 78 through diode 76, resistance 82 and diode 88, the last preventing a positive voltage from appearing across the output of the selector circuit because of the low forward diode resistance. The condenser 78 is accordingly charged with its lower terminal positive and its upper terminal negative. The negative pulse 126 efiects charging of the condenser 86 through diode 90, resistance 82 and diode 74. The result is a charging of the condenser 86 with its right hand terminal positive. However, considering the discharge circuit comprising the two condensers, and the resistances 80 and 82 and the resistance of potentiometer 92, it will be evident that the condensers 78 and 86 are charged in opposition, and consequently no net positive potential appears at the grid of the thyratron with respect to ground. Inasmuch as the cathode is maintained positive by the connection at 96, the thyratron accordingly will not be fired by the succession of pulses in the sequence B. In the case of the sequence A, the following occurs: The negative pulse 120 charges the condenser 86 through diode 90, resistance 82 and diode 74. During this charging it will benoted that the lower grounded end of potentiometer 92 is positive with respect to its upper end so that a negative signal is applied to the control grid of thyratron 94. However, after the termination of pulse 120, and while a metal particle is in the region between the two coils, the condenser 86 discharges, current flow then taking place downwardly through the potentiometer 92 giving a positive signal at the control grid of the thyratron and effecting its firing. Since a direct potential is applied to the thyratron anode, the thyratron then remains in firing condition until its anode circuit is opened through closure of switch 108 and energization of relay coil 104 which opens the contacts at 102. The firing of the thyratron energizes the coil 100 closing the contacts at 110 and thereby energizing the relay coil 112 which through contacts 116 eflect signalling and/ or controlling action.

Vibration that disturbs the coils 12 and 14 effects equal charging of the condensers 78 and 86 on alternate half cycles. Only the voltage across the condenser 86 less the voltage across condenser 78 is effective in firing the thyratron. The resistance 82 limits the charge which the condenser 86 can receive during one negative half cycle. If the frequency of the vibration is high, the positive voltage produced across the output is quite low, and accordingly, under normal vibration conditions, the arrangement is quite insensitive to vibration.

It will be evident from the above that there is provided in accordance with the invention a detecting system which is capable of distinguishing between conditions which should provide a signal and various disturbing conditions which should not do so. While of particular advantage in the case of the type of apparatus referred to, involving the possibility of metal particles in an insulating material, it will be clear that the invention is of much broader applicability where, in particular, two conditions may arise giving rise to successions of pulses such as indicated at A and B. In any such case, not only is distinction provided between such conditions, but there is also discrimination against noise and against such other conditions as produce effects which are reflected in amplitude changes as contrasted with phase shifts. It is therefore to be understood that the invention is not to be regarded as limited except as required by the following claims.

What is claimed is:

1. In combination, means for traversing material along a path, a pair of coils disposed sequentially a ong the path adjacent to the material, means providing an oscillating magnetic field through said coils, means associated with said coils providing in response to abnormal conditions of the material traversing the path a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence.

2. In combination, means for traversing material along a path, a pair of coils disposed sequentially along the path adjacent to the material, means providing an oscil lating magnetic field through said coils, phase sensitive means associated with said coils providing in response to abnormal conditions of the material traversing the path a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence.

3. In combination, means for traversing material along a path, a pair of coils disposed sequentially along the path adjacent to the material, means providing an oscillating magnetic field through said coils, means associated with said coils providing in response to abnormal conditions of the material traversing the path a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence, said detecting means including a thyratron fired upon arrival of the pulses in the first mentioned order of sequence.

4. In combination, means for traversing material along a path, a pair of coils disposed sequentially along the path adjacent to the material, means providing an oscillating magnetic field through said coils, means associated with said coils providing in response to abnormal conditions of the material traversing the path a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence, said detecting means including a pair of condensers and associated diodes through which said condensers are charged by said pulses.

5. In combination, means for traversing material along a path, a pair of coils disposed sequentially along the path adjacent to the material, means providing an oscillating magnetic field through said coils, means associated with said coils providing in response to abnormal conditions of the material traversing the path a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence, said detecting means including a pair of condensers and associated diodes through which said condensers are charged by said pulses and including a thyratron connected to said condensers and fired upon arrival of the pulses in the first mentioned order of sequence.

6. In combination, means providing a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence, said detecting means including a circuit including a pair of condensers arranged in series, and means efiecting charging of said condensers in series opposition upon arrival of the pulses in said opposite order of sequence, and efiecting charge followed by discharge of only one of said condensers upon arrival of the first of the pulses in the first mentioned order of sequence.

7. In combination, means providing a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence, said detecting means including a circuit including apair of condensers arranged in series, means effecting charging of said condensers in series opposition upon arrival of the pulses in said opposite order of sequence, and effecting charge followed by discharge of only one of said condensers upon arrival of the first of the pulses in the first mentioned order of sequence, and a thyratron fired by the last mentioned condenser discharge.

8. A combination according to claim 6 in which the last mentioned means comprises a plurality of current switching diodes.

9. A combination according to claim 7 in which the last mentioned means comprises a plurality of current switching diodes.

10. In combination, a pair of coils, means providing oscillating magnetic fields through said coils, means effecting sequential variations of said fields, means associated with said coils providing in response to said sequential variations a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence.

11. In combination, a pair of coils, means providing oscillating magnetic fields through said coils, means effecting sequential variations of said fields, phase sensitive means associated with said coils providing in response to said sequential variations a pair of electrical pulses of opposite signs, and detecting means controlled by said pulses and providing a predetermined response to said pulses upon arrival of the pulses in one order of sequence but not upon arrival of the pulses in the opposite order of sequence.

References Cited in the file of this patent UNITED STATES PATENTS 2,176,742 Pierre Oct. 17, 1939 2,209,883 Gohorel July 30, 1940 2,489,920 Michel Nov. 29, 1949 2,609,143 Stibitz Sept. 2, 1952 2,657,355 Dionne Oct. 27, 1953 2,700,149 Stone Jan. 18, 1955 2,729,809 Hester Jan. 3, 1956 FOREIGN PATENTS 464,518 Canada Apr. 18, 1950 464,880 Canada May 2, 1950 

